JPH01158813A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To realize FM one side band demodulation with no group delay caused therein within a required FM band by adopting the transversal equalizer+double limiter constitution so as to prevent inversion while applying FM equalization. CONSTITUTION:An equalizer circuit 8 consists of delay circuits 11, 12, adder circuits 9, 16, attenuators 13, 14, and a subtraction circuit 15. The delay is made constant in all frequency bands in the equalizer circuit 8 and FM equalization is applied without group delay. Then a high frequency component (k) extracted by the subtraction circuit 15 (cos equalizer) and passing through the 1st limiter circuit 3 and a low frequency component l extracted by the adder circuit 16 (-cos equalizer) and an LPF 5 are summed by an adder circuit 6, and the output signal (m) being the result of addition is given to the 2nd limiter circuit 7 to obtain an FM demodulation signal (n) with frequency equalization without any group delay. Thus, the group delay distortion in a required FM band is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM demodulation circuit, and particularly to an FM demodulation circuit that is effective for single-sideband demodulation.

[Conventional technology]

In the signal processing of conventional FM demodulation circuits installed in magnetic recording and reproducing devices such as VTRs (video recorders), there is an imbalance between the upper and lower sidebands of the reproduced FM signal read from the magnetic recording medium by the magnetic head. If this happens, the FM signal waveform of a portion of the video signal with many high-frequency components, where there is a sudden rise from black to white, will be distorted so that part of it does not reach the 0 level and oscillates. If this is applied to a limiter as it is, the zero-crossing points that carry information will be missing and the waveform will be demodulated as equivalently falling to the black level. This is called a "reversal phenomenon," and the principle of its occurrence will be explained below with reference to FIG.

Waveform a in FIG. 3(^) is the input video signal and shows a portion where there is a sudden change from black to white. When this is applied to a video emphasis circuit (not shown), the rising part of the waveform (where high frequency components are concentrated) is
A spike (overshoot) as shown in B) occurs. The FM modulator output of such a waveform a' is shown in the same figure (D).
As shown in the figure, the waveform p becomes such that the interval between the zero crossing points of the spike portion suddenly becomes narrower. If this is recorded on a magnetic recording medium such as a magnetic tape and then reproduced, the reproduced waveform will be distorted as shown in FIG. 3(E) due to the recording and reproduction mechanism. This occurs because the lower sideband components are emphasized during the recording and playback process, while the upper sideband components are suppressed. Sometimes, as shown in the figure, part of the waveform q does not reach the zero level. It can also become distorted, like it vibrates. When this is applied to the IT limiter, zero-crossing points with information are lost, and the interval between the zero-crossing points appears to be widened, so the waveform after demodulation will not be like a'.
As shown in FIG. 3C, the white level falls to the black level where it should be, causing an inversion phenomenon.

In particular, when video emphasis is applied, a spike occurs at the rising edge of the waveform (see (B) in the same figure), so this phenomenon is likely to occur. Therefore, recent home VT
In the R device, there are various measures to prevent the reversal phenomenon even if the emphasis amount is increased (for example, the DL-FM method and the reversal prevention path).

non-linear emphasis
phasis method, etc.) are being considered.

“'DL” in the DL-FM system is a double limiter (Dou
This is an abbreviation for ble Lin+1ter), which aims to prevent the reversal phenomenon by applying a double limiter and improve the SN ratio by sufficiently applying video emphasis. A specific circuit block diagram of the DL-FMM type FM demodulation circuit 1 is as shown in FIG. 2, in which 2 is an HPF (high pass filter), 4 is a phase correction circuit, and 3 and 7 are respectively 1st. The second limiter circuit, 5 is an LPF (
6 is an adder circuit.

The reproduced FM signal with waveform q supplied from the preamplifier (not shown) to the input terminal In is converted into a waveform by removing the lower sideband component corresponding to the middle to high range of the video signal by the HPF2. After creating a waveform with no missing zero-crossing points like r (see Figure 3 (G)), the first limiter circuit 3 equalizes the amplitude (removes noise near the carrier wave) and creates a waveform S (see Figure 3 (H)). ). Next, the phase distortion caused by the HPF 2 is corrected by the phase correction circuit 4, and the lower sideband wave U (see (F) in the same figure) which is the output signal of the LPF 5 is added to this by the addition circuit
The signal is added to the FM signal t to restore the distortion-free FM signal t (see (1) in the same figure), which is then passed through the second limiter circuit 7 for FM demodulation. By performing such signal processing, the SN ratio can be improved by 3 dB or more.

C Problems to be Solved by the Present Invention] In the conventional circuit 1 as shown above, HPF2 and LPF5
Since frequency division is performed using If the roll-off characteristics of each of the LPFs 5 are different, there is a problem that distortion occurs in the frequency characteristics near the cross point.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention provides a first delay circuit that delays an input signal by a predetermined time.

a second delay circuit that further delays the output signal of the first delay circuit by the same delay time; a first addition circuit that mixes the output signal of the second delay circuit with the input signal; and a first addition circuit. A -cos equalizer is configured by a first attenuator that attenuates the output of F by a predetermined amount, and a subtraction circuit that subtracts the output of the first attenuator from the output of the first delay circuit.
M equalization, and a second attenuator that similarly attenuates the output of the first addition circuit by a predetermined amount, and a second addition circuit that adds the output of the second attenuator to the output of the first delay circuit. cos
Configure an equalizer to FM equalize the input signal, and -c
A first limiter circuit that limits the amplitude of the output of the OS equalizer, a low-frequency filter that passes only low-frequency components of the output signal of the COS equalizer, a low-pass filter, and the output of the first limiter circuit. By further comprising a third adder circuit for mixing and a second limiter circuit for limiting the amplitude of the output of the third adder circuit (i.e., transversal equalizer + double limiter), within the required FM band. It is configured to realize single sideband demodulation that suppresses group delay distortion and prevents inversion.

〔Example〕

The FM demodulation circuit of the present invention has a transversal equalizer and double limiter configuration as described above, and
FM that prevents inversion while performing M equalization and does not cause group delay.
This is realized within the FMM range required for one sideband demodulation, and an embodiment of the FM demodulation circuit of the present invention will be described below with reference to FIG. Figure 1 shows the FM of the present invention.
1 is a block configuration diagram of the main parts of a demodulation circuit 10. FIG. In this figure, the same components as those in the conventional example shown in FIG. 2 are given the same reference numerals, and detailed explanation thereof will be omitted. In the figure, 1
1.12 is a delay circuit, 9 and 16 are adder circuits, 13.1
4 is an attenuator, 15 is a subtraction circuit, and the above configuration constitutes the equalizer circuit 8.

Next, the principle of operation of the FM demodulation circuit 10 will be explained with reference to FIG. 1 and FIGS. 4 onwards. The Wt input signal a supplied to the input terminal In of the FM demodulation circuit 10 is a=E e , and the delay circuit 11.12
Assuming that the delay time of
+2τ). Therefore, the output d of the adder circuit 9 is d
=a0c=E(60ejW(t+2r).

wt = E e jW (10τ) −jwτ
JWτ + (e 10e = 2EejvI”” Gos wτ・・・・・・・・・
......(1).

The attenuation factors of attenuators 13 and 14 are α and β (usually α, β≦
temple), then the output elf is e=2aEe jw ”10τ) cos w v
−0−−0−−−−11,−(2)f=2βE e j
w(t+f)CO8w τ−−−−−−−−−−−−
--(3), so the subtraction circuit 15 and the addition circuit 16
The outputs g and h of are respectively g=be-e=Ee"(t"")-2aEe""τ)cosw'r=E (1-2a C03wτ) e"(t+"-(4
) h=b+t=Eejv4(t+f) +2βE e ” (t+” )CO8Wτ=F, (1
+2βcoswτl e jW4(j ” r ) +
++ (1) From equation (4), the following can be seen regarding the output signal of the subtraction circuit 15.

Although the O output signal has a time delay with respect to the input signal, there is no phase distortion at all.

The frequency characteristics for zero amplitude (gain) are as shown in FIG.

Such characteristics are called cos (cosine) characteristics.

Similarly, from equation (0), the output signal of the adder circuit 16 also has a time delay with respect to the input signal, but there is no phase distortion at all, and the frequency characteristic with respect to amplitude becomes a -cos characteristic as shown in FIG.

Center frequency chi of an equalizer with delay time τ. (W=2π
ChiC) is Chi. =1/2τ. In this equalizer circuit 8, the delay is constant over the entire frequency band, and FM equalization can be performed without group delay. Subtraction circuit 1
The characteristics of the output signal of No. 5, that is, the equalized FM wave, are:
As mentioned above, the transfer function is as shown in Figure 4, so from y1=-αχ(1+zdo')+2ZT-'), Hl(jω)=(1-2acosθ)ε-jθ...
■However, zT-' = 1/2 fc (delay amount)
θ = 2 x f / 2 fc - π
As is clear from the formula (■), the amplitude (1-2αc
osθ) and phase ε-jo, there is no group delay. A signal g having such a waveform is supplied to the first limiter circuit 3. The amount of lift is controlled by the gain α of the attenuator 13, and this becomes the amount of high-frequency component addition.

Further, the characteristics of the output signal (FM wave) h of the adder circuit 16 are as shown in FIG. That is, the transfer function is y2=βx (
1+ZT-') +x Z7-'), H2(jω)=(1+2βcosθ)ε-jθ...■
However, θ−πf/f.

As is clear from equation (2), the amplitude (1+2β cos θ) and the phase ε-jo result in a cos equalization curve with the center frequency set at f without group delay, and the output signal of such a waveform is set at the frequency t0 as the cutoff point. By passing the signal through the LPF 5 which has undergone group delay correction within the pass band, it becomes a low-frequency component with frequency characteristics as shown by curve E in FIG. 6 (curve B is the same as in FIG. 4).

Note that the amount of low-frequency component addition is controlled by the gain β in the attenuator 14. That is, the high-frequency component extracted by the subtraction circuit 15 (cos equalizer) and passed through the first limiter circuit 3, the addition circuit 16 (-cos equalizer), and the frequency t. LPF5 with cutoff point
The adder circuit 6 adds the low-frequency components i extracted by The signal is obtained from the furnace and outputted to a downstream FM detector (not shown) or the like. In this way, a frequency-equalized double limiter circuit without group delay can be constructed within the required FM band.

〔effect〕

Since the FM demodulation circuit of the present invention is constructed as described in detail above, it has various practical effects as described below.

■It has become possible to reduce group delay distortion within the required FM band, which was difficult with conventional FM demodulation circuits.

(2) In conventional FM demodulation circuits, it was difficult to reduce frequency distortion and group delay distortion near the cross point between high and low frequency components, but this has become possible with the circuit of the present invention.

■The circuit can be simplified by using a transversal equalizer + double limiter configuration.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the main parts of the FM demodulation circuit of the present invention, Fig. 2 is a conventional circuit diagram, Fig. 3 is a signal waveform diagram for explaining the operation of the conventional circuit, and Figs. FIG. 3 is a signal waveform diagram for explaining the operation of the inventive circuit. 3... First limiter circuit, 5... LPF, 6.9°16... Adder circuit, 7... Second limiter circuit, 8...
. . . Equalizer circuit, 10 . . . FM demodulation circuit, 11.
12...Delay circuit, 13.14...Attenuator, 15.
...Subtraction circuit. Patent Applicant: Victor Company of Japan = 13- Star 3 Diagram Current Illustration 6 Procedural Amendment

Claims (1)

[Claims] An FM demodulation circuit that demodulates an FM modulation signal includes a first delay circuit that delays an input signal by a predetermined time;
a second delay circuit that further delays the output signal of the delay circuit by the same delay time; a first addition circuit that mixes the output signal of the second delay circuit with the input signal; and the first addition circuit. a first attenuator that attenuates the output of the first attenuator by a predetermined amount;
A subtraction circuit that subtracts the output of the first attenuator from the output of the delay circuit constitutes a -cos equalizer to FM equalize the input signal, and similarly adjusts the output of the first addition circuit to a desired amount. A cos equalizer is configured by a second attenuator that attenuates the input signal by a second attenuator, and a second addition circuit that adds the output of the second attenuator to the output of the first delay circuit to FM equalize the input signal. , a first limiter circuit that limits the amplitude of the output of the -cos equalizer; a low-pass filter that passes only low-frequency components of the output signal of the cos equalizer; By further comprising a third adder circuit that mixes the output of the limiter circuit and a second limiter circuit that limits the amplitude of the output of the third adder circuit, group delay distortion can be suppressed within the required FM band. An FM demodulation circuit characterized in that the FM demodulation circuit is configured to realize one sideband demodulation that prevents inversion.